Leucoindigo



' process be uneconomical.

Patented Sept. 20, 1938 Alfred Davidson and Colin Henry Lumsden,

Blackley, Manchester, England, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application April 6, 1934, Serial No. 719,410. In Great Britain April 10, 1933 Claims.

This invention relates to indigo dyestuffs. More particularly the invention relates to leucoindigo and to a method of preparing it.

One of the most important of all indigo com- -5 pounds is leucoindigo, a compound which afiixes itself readily to fibers and is readily oxidized to indigo on exposure to the air.

It is known that indoxyl and indigo can interact with formation of leucoindigo as indicated in the following equation:

and further that indoxyl can be oxidized by air or other oxidizing agents at a suitable temperature in such a manner that-the product is substantially leucoindigo, any indigo formed by the oxidation of the indoxyl presumably interacting with indoxyl as described above. It has been proposed in British Patent 6,226 of 1904 to utilize these reactions for the manufacture of leucoindigo directly from indoxyl, thus saving part of the cost of oxidizing indoxyl to indigo and then reducing indigo, e. g. by sodium hydrosulfite, to leucoindigo.

But unless the caustic alkali used in producing the indoxyl can be separated from the leucoindigo in such a form as to be recoverable for repeated use, the above-mentioned saving may easily be outweighed by the loss of this alkali, and the No method of recovery of the alkali is described in the above-mentioned British patent.

Separation of the indoxyl from the alkali may, however, be accomplished as described in British 40 Patent 22,288 (1910), and the separated indoxyl could then be treated as proposed in British Patent 6,226 (1904) so as to form leucoindigo. But this method sufiers from the disadvantage that the comparatively unstable indoxyl is subjected to 45 a somewhat lengthy preliminary treatment, during which it is liable to decompose forming colored by-products which contaminate the leucoindigo.

An object of the present invention is to prepare leucoindigo .by the direct interaction of an alkaline indoxyl melt with indigo. Another object of the invention is to overcome the disadvantages which attended this process according to the methods used in the prior art. Another object of the invention is to prepare leucoindigo of superior purity. Another object of the invention is to prepare leucoindigo by a process which permits the recovery of the caustic alkali used therein.

The objects of the invention are accomplished, generally speaking, by reacting a caustic alkaline indoxyl melt directly with an aqueous suspension of indigo, adjusting the proportion of water in the caustic alkaline solution, precipitating the leucoindigo more or less in the form of its alkali metal salts and by freeing the leucoindigo by reacting the said salts with suitable reagents. The details of the methods hereinafter described also assist in accomplishing the objects of the invention to the degree hereinafter specified.

We have found that leucoindigo is precipitated as an alkali metal salt from its aqueous caustic alkaline solutions if the concentration of the caustic alkali exceeds a certain minimum amount, roughly about 15% of the weight of the solution in the case of caustic soda, and that the proportion of the leucoindigo so precipitated increases rapidly with increased concentration of the caustic alkali, so that when the alkali concentration reaches 22-23% about 75-80% of the leucoindigo is precipitated. As a result we find it is possible to cause a caustic alkaline indoxyl melt (produced in the normal way by fusing phenylglycine with caustic alkali and sodamide) to interact directly with an aqueous suspension of indigo, and then by adjusting the proportion of water in the caustic alkaline solution, precipitate the leucoindigo in large measure as an alkali metal salt, filter this and treat the filtrate to recover alkali.

Working in this way, however, we find that the precipitation of the alkali metal salt of the leucoindigo is liable to occur with formation of amorphous tarry masses which occlude impurities rendering the dyeing shade of the final product dull. This, we find, is due in part to the presence of certain aromatic bases, such as aniline and methylaniline, which are always produced as byproducts in the indoxyl melt. We, therefore, carry out the interaction of the indoxyl melt with the indigo at greater aqueous dilution than would be suitable for direct precipitation of the leucoindigo and then, by evaporation of the excess of water, simultaneously remove the aniline, etc. and adjust the alkali concentration to the required degree. In this way it is possible to precipitate the leucoindigo salt in relatively pure form as yellow needle-shaped crystals, which are easily filterable.

If, however, the caustic alkali employed in the indoxyl melt consists, as is usual, of a mixture of caustic soda and caustic potash, the precipitated leucoindigo salt, which in this case consists of a mixture of the sodium and potassium salts, may not be well crystallized if the proportion of potassium hydroxide exceeds about 50% by weight of the total hydroxide present in'the mixture. We, therefore, find it advantageous to adjust, if'necessary, the proportions of sodium and potassium hydroxides present in the final reaction mixture by addition of small quantities of a highly concentrated aqueous solution of sodium hydroxide or of a mixture of sodium and potassium hydroxides containing a higher proportion of the sodium compound than that present in the reaction mixture. This addition may be used in part in assisting the adjustment of the mixture to the desired concentration for precipitation of the leucoindigo. It is not, however, a necessary feature of our invention, but assists in the production of a leucoindigo in good crys-v Example A mixture of indoxyl (850 parts), sodium hydroxide (1700 parts) and potassium hydroxide (1250 parts) together with the usual by-products, produced by the fusionof phenylglycine with caustic alkali and sodamide, is run in a molten conditioninto a well-agitated suspension of 855 parts of indigo in 10,000 parts of cold water. Cooling is applied, if necessary, so as to adjust the final temperature of the mixture to -95 C. and the mixture is maintained at thisv temperature, while stirring,'for 15 minutes. A solution of leucoindigo in caustic alkali is thereby produced. Under diminished pressure water is distilled from the solution during 30-60 minutes until the distillate no longer contains oily droplets of aniline or methylaniline and the total weight of distillate is about 1200 parts. The solution is then cooled, during 1 to 2 hours, to 10 C. and, while cooling, 1500 parts of a 50% solution of sodium hydroxide are added. After stirring for about hour at 10 C., the resulting suspension of crystals of the sodium and potassium salts of leucoindigo in caustic alkali solution is filtered. The filter-cake of leucoindigo salt is stirred into 9000 parts of warm water in which it dissolves completely except for a small quantity of indigo. The latter is removed by filtration and the solution is then neutralized by gradual addition of about 3400 parts of a 20% solution of sulfuric acid. Indigo white is thus precipitated and, after filtration and.

Washing to remove dissolved salt/8, 63.11 be employed in the usual manner. s

An advantage of this invention is a new process for producing leucoindigo. Another advantage is the production of leucoindigo in a very pure form. Another advantage of the invention is that the alkali used in the process can be recovered almost quantitatively. Other advantages of the invention will be apparent to those skilled in the art. a

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments thereof except as defined in the appended claims.

We claim:

A 1. The method of preparing leucoindigo which comprises mixing a phenylglycine fusion mass containing indoxyl with caustic alkali and indigo in excess of Water, removing excess of aniline, methylaniline and other vaporizable indigenous impurities by evaporation, precipitating leucoindigo in the form of its alkaline salts by raising the alkali concentration to an amount at least equivalent in efiect to about 15% of sodium hydroxide, removing the precipitate, dissolving it'in water, and adding to the solution acid sulficient toprecipitate the leucoindigo as indigo white.

2. The method of preparing leucoindigo which comprises mixing a phenylglycline fusion mass containing indoxyl with caustic alkali and indigo in Water, distilling off the vaporizable impurities at subatmospheric pressure, and increasing the concentration of caustic alkaliuntil the leucoindigo is precipitated as its alkaline salt, removing and dissolving in water the leucoindigo. salts, neutralizing the solution with acid, and separating out the indigo white.

3. The method of preparing leucoindigo which comprises mixing a phenyl glycine fusion mass containing indoxyl, sodium hydroxide and potassium hydroxide with indigo and water, fixing the temperature of the mix at about 90 to about C., distilling 01f vaporizable impurities at subatmospheric pressure, cooling, adding caustic alkali to bring the weight thereof to an amount equivalent in efiect to at least 15% sodium hydroxide, separating and dissolving the precipitate in warm water, filtering the solution, and adding thereto enough acid to precipitate indigo white.

4. The method of preparing leucoindigo which comprises reacting a phenyl glycine fusion mass containing about 850 parts of indoxyl, about 1700 a parts ofsodium hydroxide, about 1250 parts of potassium hydroxide and indigenous impurities with about 855 parts of indigo in about 10,000

parts of water, fixing the temperature of the mixture at about 90 to about 95 C., distilling at sub-atmospheric pressure until the distillate is substantially free from aniline and methylaniline and until the distilland has a total weight of about 1200 parts, cooling to about 10 C., adding about 1500 parts of 5% aqueous sodium hydroxleuco-indigo to a total alkali content in excess of 22 per cent, expressed as 'NaOH,'cooling tocrystallize alkali metal salt of leuco-indigo, and separating the crystals. 3 a

' I ALFRED D'AvInsoN. I COLIN. HENRY LUMSDEN.-

- CERTIFICATE OF CORRECTION.

atent No.- 2,1 0,878, September 20, 193

ALFRED DAVIDSON, ET AL.

It is hereby certified that error appears in the printed specification if theabove numbered patent requiring correction as follows: Page 2, second :olumn, line 5?, claim h, for "5%" read 50%; and that the said Letters Patent :houldbe read with this correction therein that the same may conform to :he record of the case in the Patent Office. I I

Signed and sealed this 25th day of October, A, Do 19 8.

Henry Van Arsdal-e (Seal) Acting Commissioner of Patents.) 

